SX-780 Bright bulb on DBT - Looking for help...

[dibmem]

Not out of the ordinary for me - I've had to tear down and clean the contacts on a power switch (SX-550) before. I'll look into that.
Here's the current set of measurements:
Q25
Pin 13 (+50V?) 44.4v
12 + 13 V (25?) 22.7v

Q19
R317 (top) 44.4v (bottom) 43.3v
Q20
R318 (top) -45.6v (bottom) -44.7v

C307 + side (+39.5?) 40.1v
C308 - side (-39.5) 40.2v (I get -40.2v on + side of C308 - if I'm following the schematic correctly)

R301: I can't find it LOL
R302: (+23.7v?) 24.4v (top) / 22.4v (bottom)
R304: (-23.7v?) -24.0v (top) / -21.0v (bottom)
R301: I can't find it - looking in the schematic...

 

[mrk229]

Not out of the ordinary for me - I've had to tear down and clean the contacts on a power switch (SX-550) before. I'll look into that.
Here's the current set of measurements:
Q25
Pin 13 (+50V?) 44.4v
12 + 13 V (25?) 22.7v

Q19
R317 (top) 44.4v (bottom) 43.3v
Q20
R318 (top) -45.6v (bottom) -44.7v

C307 + side (+39.5?) 40.1v
C308 - side (-39.5) 40.2v (I get -40.2v on + side of C308 - if I'm following the schematic correctly)

R301: I can't find it LOL
R302: (+23.7v?) 24.4v (top) / 22.4v (bottom)
R304: (-23.7v?) -24.0v (top) / -21.0v (bottom)
R301: I can't find it - looking in the schematic...

Those voltages look good. Have to take into account the voltage drop of the Bulb and the potential drop at the power switch.

The + side of C308 should be at ground potential. Don't sweat R301, R302 has good voltage so R301 should too, unless it is open.

You don't by chance have a Kill A Watt do you?


I use one on the output of my DBT. You can see the actual voltage, current and power used in watts. Very handy too see if you have a high draw, numbers instead of how much glow is in the bulb.

I think you may have a high draw. At idle with no audio and only one STK pack I would think a 100 watt bulb would be nearly extinguished.

Is the remaining power pack getting warm at all?

Does the protection relay click in after a short delay?

 

[dibmem]

No relay click still - I'll leave it on and check the temp on the other STK in a few minutes.
Kill a watt in line with DBT: 119v / 32.9VA / 29.9 watt / .27 Amp
The 100 watt bulb is nearly extinguished when the 780 is on.

In case we need numbers:
STK installed
1 22.6v
2 -34.3v
3 -34.3v
8 0
9 34.5v
0 25.1v

STK (empty pads)
1 -39.7v
2 -34.3v
3 0v
8 0v
9 34.5v
0 35.2v

After 10-15 minutes at idle, the installed STK doesn't go above 85 degrees.

 

[mrk229]

Since the relay is not pulling in we need to determine why.

1. High offset voltage. I don't recall which STK pack is still installed but the image below shows both points where you can check the offset voltage.
See what the DC voltage is on the point with the installed STK. If it is not in the low millavolt range there is a problem in the amplifier channel.
If it is in the low millavolt range then move to step 2.



2. The problem is likely in the protection circuit.
Check for the +13.5 volts that powers the IC, either at the emitter of Q25 or pin 1 of IC Q27.
If the voltage is present then check for voltage on both sides of R321.

 

[dibmem]

Below is a photo of current status. I am taking measurements at wall voltage. If I should install resistors on the missing power pack, I have them ready - although I may not get to continue until Monday ;-)

Here's some measurements:
Pin 4: -40.4v
Pin 9: -23.8mV

Q25: e 13.4v / b 14.1v / c 26.7v
R321: 50.9v on top and bottom of that resistor (faulty?)

I also pulled all the solder off the legs of Q25, Q19, and Q20 and added fresh shiny solder to ensure things are solid.
Tested the transistors (with a DCA55) while I had them disconnected and all seemed to pass, but I'm not sure - the numbers for all of those are available if needed.

 

[mrk229]

Below is a photo of current status. I am taking measurements at wall voltage. If I should install resistors on the missing power pack, I have them ready - although I may not get to continue until Monday ;-)

Here's some measurements:
Pin 4: -40.4v
Pin 9: -23.8mV

Q25: e 13.4v / b 14.1v / c 26.7v
R321: 50.9v on top and bottom of that resistor (faulty?)

I also pulled all the solder off the legs of Q25, Q19, and Q20 and added fresh shiny solder to ensure things are solid.
Tested the transistors (with a DCA55) while I had them disconnected and all seemed to pass, but I'm not sure - the numbers for all of those are available if needed.

View attachment 3781515

Actually that is all encouraging. R321 has essentially the same voltage on both ends because the relay is not pulled end and there is no load.

I would check the voltage at the collector of Q26 to confirm the relay coil is good. Then I would pull and check Q26. If Q26 checks good it is possible then we need to look at the voltages at the protection IC starting with pin 6.